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Pertungstic Acid, H2WO5

By the action of hydrogen peroxide on solutions of tungstic acid and tungstates, compounds analogous to the persulphuric acids and persulphates are obtained. By dissolving tungsten trioxide in a 2 per cent, hydrogen peroxide solution at the boiling-point, and evaporating the resulting solution, a yellow amorphous mass results, of composition WO3.H2O2.H2O and soluble in water to an acid solution.

Several sodium salts have been prepared:

Na2W2O8.2H2O is obtained as white radiating crystals by the addition of hydrogen peroxide to a hot solution of sodium paratungstate and evaporation of the mixture in a vacuum. The crystals are easily soluble in water.

Na2W2O9.6H2O results as a yellowish-white crystalline powder when the above mixture of paratungstate and hydrogen peroxide is precipitated with alcohol. In this salt the ratio of WO3 to active oxygen, as determined by titration with potassium permanganate in sulphuric acid solution, is 1:1, whilst in the above dihydrate it is 2:1; this suggests that the latter is derived from an acid of constitution 2WO3.H2O2, i.e. H2W2O8, and the former from an acid of constitution WO3.H2O2, i.e. H2WO5, of which H2W2O9 is an anhydro-acid (i.e. 2H2WO5 - H2O ).

By the addition of cooled hydrogen peroxide and aqueous sodium hydroxide solutions to a cooled solution of sodium pertungstate, prepared according to the former method described above, precipitating with alcohol at -12° C. and drying the resulting product in the air, compounds of composition Na2O2.WO4.H2O2 and Na2O2.WO4. H2O2 + (Na2O2)2WO4 + 7H2O have been obtained. These are unstable compounds, although more stable than the corresponding molybdenum compounds; they gradually lose oxygen on standing in air, whilst water causes rapid evolution of oxygen, hydrogen peroxide being found in the resulting solution.

The potassium salt, K2O4.WO4.H2O, is similarly produced by the action of hydrogen peroxide and potash on potassium pertungstate. It is more stable than the sodium salts, but explodes when rubbed or heated to 80° C.

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